The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the inventors hereof, to the extent the work is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, is neither expressly nor impliedly admitted as prior art against the present disclosure.
Memory cells, especially NAND flash memory cells, store bits by storing different amounts of charge in the floating gate of a transistor, and accordingly exhibit different amounts of threshold voltage. Additionally, noise influences the cells containing the same stored bit to have different threshold voltages. Because of these issues, memory cells do not have a fixed threshold voltage, but rather form a distribution of threshold voltages. Current flash memory devices do not provide the capability to directly measure their threshold voltages. However, the inherent threshold voltage of a memory cell is required to be known in order to correctly read the bits stored in these memory cells.
The current methods for determining the soft information of memory cells, an estimate of the inherent threshold voltage of the memory cell, are neither robust nor efficient. Current methods probe a memory cell with a sequence of monotonically increasing or decreasing reference voltages in a multi-pass read. However, if one of the applied reference voltages in such a multi-pass read falls out of the monotonically increasing or decreasing order, then the soft information generation method is corrupted. This corruption occurs because current methods confuse the voltage value of the out of order reference voltage with the voltage value of the expected reference voltage. Such a mistake leads to corruption of the soft information generation method because the measured output of the memory cell is not associated with the correct reference voltage value. In addition, due to read-to-read variation in actual memory devices, which result from measurement inaccuracies and noise affecting the read operation, the measured output from one cell may contradict with a previous read from the same cell. Current methods are not capable of handling such inconsistencies.